Transformer tap selector

ABSTRACT

Tap selecting means providing minimum flux symmetry destruction at different tap settings. The tap selecting means includes movable and stationary contacts connected to tap terminals on coils of the transformer winding structure. The contacts and the tap connections are arranged to provide maximum tapout in one coil before tapping-out another coil.

United States Patent [191 Thomas 51 Apr. 3, 1973 [541' TRANSFORMER TAPSELECTOR 3,644,786 211972 Yannucci ..336/150 x 1,873,824 8 1932 C l t l. .336 145 [75] Inventor: Michael W. Thomas, Muncie, Ind. o e e a l [73] Assignee: Westinghouse Electric Corporation, Primary Examineb'Thomas Kolma Pittsburgh Pa Attorney-A. Stratton et a].

[22] Filed: Apr. 28, 1972 [57] ABSTRACT [21] Appl. No.: 248,474 Tap selecting means providing minimum flux symmetry destruction at different tap settings. The tap selecting means includes movable and stationary con- [52] 0.8. CI. ..336/l46, 323/435, 336/150 v was connected to tap terminals on coils of the "arm [51] II.- Cl. ..H0ll' 21/12 former i i structure; The contacts and the tap [58] Field of Search ..323/43.5; 336/150, 145, 146 connections are arranged to provide maximum tapout in one coil before tapping-out another coil.

[56] References Cited 6 Claims, 5 Drawing Figures UNITED STATES PATENTS 857,062 6/1907 l-Iilliard.'..... ..336/145 X PATENTE'BAPM ma SHEET 3 [1F 5 1 TRANSFORMER TAP SELECTOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates, in general, to electrical inductive apparatus and, more specifically, to transformer tap changers.

2. Description of the Prior Art No-Ioad tap'changers are used in power transformers to change the rating of a winding of the transformer. Usually, the tap changer changes the number of winding turns which are electrically active within the winding. In large power transformers, such as high-power shell-form transformers, it is important to keep losses due to leakage flux at a minimum. This requirement makes it desirable to change the taps in a shell-form transformer winding in a manner which produces the lowest amount of leakage flux and power loss.

To minimize leakage flux losses, it is important to maintain maximum symmetry of the conductors in the winding. In shell-form transformers, the windings consist of winding sections having a plurality of pancake type coils each spirally wound with several turns of a conductor. The tap selector leads are connected to conductor-turns within the coils and to tap selecting means which interconnects the leads together. A predetermined number of coil turns may be tapped-out by interconnecting specific leads or tap terminals together. I

Leakage flux which is perpendicular to a coil at a particular point in the coil is substantially a function of the positionof the point relative to the height or linear build of the coil. At the center of the linear build of the coil, the perpendicular, or axial, leakage flux is substantially zero. The axial leakage flux increases, with one polarity, when the point of measurement is moved in one direction from the center of the coil height. The

axial leakage flux increases, with the opposite polarity,

when the point of measurement is moved in the other direction from the center of the coil height. When every turn of the coil is electrically in the circuit, the net axial flux across the conductor of the coil is substantially equal to zero. As a result, the eddy and stray losses of the transformer, which are dependent upon the leakage flux, are at a minimum.

When only part of the total turns of a coil are electrically in the circuit, such as when some coil turns are tapped-out by the tap selecting means, destruction of the flux symmetry within the coil occurs. According to arrangements used in the power art, the destruction of symmetry increased the losses of the winding and decreased the efficiency of'the transformer. Normally, the desired tapping range requires that more than one coil have tap terminals connected thereto. Thus, in certain tap selecting positions, a number of turns from more than one coil must be tapped-out. When the tapped-out portion of the coils occupy substantially the same radial position in the coils, the destruction of symmetry inthe winding is compounded.

In removing conductor-turns from a winding, prior art arrangements have progressively tapped-out part of a first coil, part of a second coil, more of the first coil, and then more of the second coil because of the simplicity of the tap selecting switch required to make tap changes according to this sequence. Since transformers are usually rated at the center tap position, that is, at a rating which can be raised or lowered by the tap selector, the transformer losses are specified at the center tap selecting position. Hence, with the unsymmetrical arrangements known in the prior art, the transformer efficiency was influenced by the losses occasioned by the lack of coil turn symmetry at the center or rated position. 7

It is advantageous to reduce the transformer losses to a minimum when in different tap positions and to reduce the losses in the rated or guaranteed tap selecting position. Therefore, it is desirable, and it is an object of this invention, to provide a tap selecting means which produces the maximum flux symmetry at the 5 center tap selecting position.

SUMMARY OF THE INVENTION There is disclosed herein new and useful apparatus for selecting tap positions in a transformer winding structure. The tap selecting means includes movable and stationary contacts which are connected to tap terminals in the winding structure. The contacts are located andconnected to the tap terminals in a manner which taps-out conductor-turns of the winding coils in a sequence which provides minimum flux symmetry destruction at the center tap selecting position. At the center position, all of the conductor-turns tapped-out of the winding are physically positioned in one coil. When the tap position is lowered to reduce the voltage rating of the winding, additional conductor-turns are tapped-out of another coil. When the tap position is raised from the center position to increase the voltage rating of the winding, less conductor-turns are tappedout of the same coil. The contacts of the tap selecting means are arranged to provide this tapping sequence with a minimum of contacts and without complex mechanisms.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of this invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is a view of a shell-form power transformer having a tap selector constructed according to an embodiment of this invention;

FIG. 2 is a schematic diagram of a tap selector constructed according to an embodiment of this invention and connected to adjacent coils;

FIG. 3 is a schematic view of a symmetrical flux pattern in a winding;

FIG. 4 is a schematic view of an asymetrical flux pattern in a partially tapped-out winding according to a prior art arrangement; and

FIG. 5 is a schematic diagram of a tap selector constructed according to another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS tap selector constructed according to an embodiment of this invention. The transfonner casing supports the hightvoltage bushings 12 and 14 and the low-voltage bushing 16. The laminated magnetic core 18 of the transformer is of the shell-form type with a portion of the magnetic core 18 positioned adjacent to the walls of the casing 10. The transformer comprises a plurality of winding sections, such as the high-voltage winding sections 20 and 22 and the low-voltage winding sec tions 24 and 26, each of which are shown with insulating members disposed thereon.

The winding sections 20, 22,24, and 26 contain a plurality of coils each spirally wound with a conductor. Tap leads 28 are connected to the coils of the high-voltage winding sections 20 and 22 and to the tap selecting means 30. The tap selecting means 30 provides means for changing the voltage rating of the high-voltage winding assembly when not under load.

The tap selecting means 30 includes a plurality of stationary contacts 32 which are mounted on an insulating support member 34. A movable contact 36 is rotated by the shaft assembly 38 which is coupled, by the universal joint 40,- to a wheel or other means on a wall of the transformer casing 10. The wheel may be turned to rotate the movable contact 36 for selecting a different tap position. The setting of the tap selecting means determines the voltage rating of the high-voltage winding assembly. The-movable contact 36 is rotatable to a number of fixed positions, each position connecting together two of the stationary contacts 32.

FIG. 2 is a schematic diagram of the tap selecting means 30 and its electrical connections to the highvoltage coils. A first stationary contact 41, a second stationary contact 42, a third stationary contact 43, a fourth stationary contact 44, a fifth stationary contact 45, and a sixth stationary contact 46 are positioned as illustrated on the insulating support member 34. The movable contact 36, which is shown in phantom above the stationary contacts 41 and 44, is attached to means which can be used to change the position of the movable contact 36. The stationary contacts 41, 42, 43, 44, 45 and 46 are connected to the tap terminals 51, 52, 53, 54, 55 and 56, respectively. The tap terminals 51, 52, and 53 are connected to the conductor-turns of the coil 58 and the tap terminals 54, 55 and 56 are connected to the conductor-turns of the coil 60.

The coils.58 and 60 are inductively coupled to the magnetic core 18. Coils 62 and 64 are similarly coupled to the magnetic core 18. The coils 58, 60, 62 and 64 are shown in a schematic arrangement which illustrates the location of the magnetic core 18, the relative position of the coils, and the electrical position of the tap terminals 51, 52, 53, 54, 55 and 56. Conductor-turns, such as 65, are radially disposed upon adjacent conductor-turns from the start end to the finish end of the coils, such as theends 66 and 68, respectively, of the coil 58.

The first tap terminal 51 and the fourth tap terminal 54 are connected to finish conductor-turns of the coils 58 and 60, respectively. The second tap terminal 52 and the fifth tap terminal 55 are connected to conductor-turns between start and finish coil turns. The third tap terminal 53 and the sixth tap terminal 56 are connected to conductor-turns located between the other tapped conductor-turns. When two tap terminals on different coils are electrically connected together by the tap selecting means 30, some of the conductortums may be removed from the winding circuit. For example, when the tap terminals 52 and 54 are electrically connected together, the conductor-turns between the tap terminals 51 and 52 are bypassed. Thus, the current flowing in the coil 58 is confined to the region between the tap terminal 52 and the end 66.

It is desirable to have the net flux which is perpendicular to a coil equal to zero to make eddy currents in the conductors as low as possible. Without any portion of the coils 58 or tapped-out, the net axial flux produced by these coils and the other coils of the winding assembly is substantially equal to-zero. The axial flux changes polarity near the center of the coils and the net axial flux measured across the radial direction of the coil, or across the linear build thereof, is substantially equal to zero.

FIG. 3 is a diagram illustrating the flux pattern which exists perpendicular to the radial direction of the coils 58, 60, 62 and 64. The diagram of FIG. 3 represents the flux pattern when all of the coils are completely in the circuit. This condition exists when the tap terminals 51 and 54 are connected together as indicated by the connection 70. This provides the highest voltage rating for the winding and corresponds to the position of the movable contact 36 shown in phantom in FIG. 2[

In FIG. 3, the flux lines 72 represent the amount of flux in one direction. The flux lines 74 represent the amount of flux in the opposite direction. The line 76 represents the center of the coils where the flux is zero. The conductors of the coils spiral from one end of the coil to the other end, thus all of the flux indicated by the lines 72 and 74 is perpendicular to the coil conductor. InFIG. 3, the net axial flux is zero since the amount in one direction is equal to the amount in the other direction. Therefore, the amount of eddy current due to leakage flux is low.

FIG. 4 illustrates a flux pattern which exists when the same ends of adjacent coils are tapped-out. This pattern exists with a prior art arrangement when the tap terminals 53 and 56 are connected together as indicated by the connection 78. This connecting arrangement provides the center, or guaranteed, voltage rating of the winding. The coils 58 and 60 are hatched to illustrate that a portion of one end of the coils are not producing flux. The resulting flux is indicated by the lines 80 and 82. Line 84 represents the region where the flux is zero and may not necessarily be located at the center of the coils. Since the net flux is not equal to zero, eddy currents are induced into the coil conductors which extend between the ends of the coils. Consequently, losses are substantially high at this tap position according to the prior art arrangement.

In the tap selecting means constructed and connected according to this invention, symmetry throughout the tap range is maintained as high as possible with the number and position of tap terminals as indicated. Referring again to FIG. 2, the movable contact 36 is shown in the high position. At this position, stationary contacts 41 and 44 are connected together.

This efiectively connects together the tap terminals 51 and 54. Symmetry'is maintained since each winding section is completely in the circuit.

When the movable contact 36 is rotated to the next position, a voltage rating below the highest voltage rating is produced. The stationary contacts 43 and 44 are electrically connected together which effectively connects together tap terminals 53 and 54. With this arrangement, destruction of symmetry is caused only by the one coil 58. When the movable contact 36 is rotated to the next position, the center or nominal voltage rating is produced. The stationary contacts 42 and 44 are connected together which effectively connects together the tap terminals 52 and 54. The destruction of symmetry with this arrangement depends upon the radial position of the tap terminal 52 within the coil 58. If the tap terminal 52 is at the end 66 of the winding section 58, symmetry is restored. If the tap terminal 52 is between the end 66 and the center of the coil 58, destruction of symmetry may be reduced in the center position by this arrangement. In any event, the destruction of flux symmetry in the center position is less pronounced with the arrangement taught by this invention compared to the prior art arrangement.

When the movable contact 36 is moved to the next lower position, the voltage ratingis reduced. The stationary contacts 42 and 46 are electrically connected together which effectively connects together the tap terminals 52 and 56. With this arrangement, the portion of the coil which is between the tap terminals 51 and 52 is tapped-out and the portion of the winding 60 which is between the tapterminals S4 and 56 is tappedout. In the lowest voltage rating position, the movable contact 36 connects the stationary contacts 42 and 45 together. This effectively connects together the tap terminals 52 and 55 which taps-out the portions of the coils 58 and 60 which are between the tap terminals 5] and 52, and 54 and 55, respectively.

The contacts of the tap selecting means 30 may have any shape suitable for making the desired connections and providing the necessary current capacity. The semicircular contacts 42 and 44, as illustrated in FIG. 2, have projections 86 and 88 thereon, respectively, for increasing the current capacity. Although shown having a substantially triangular shape, the movable contact 36 may have other shapes, such as square, rectangular, or circular. The stationary contacts 41, 43, 45 and 46 are shown located within the circle formed by the semicircular contacts 42 and 44. It is within the contemplation of this invention that they may also be located on the outside periphery of the circle formed by the semicircular contacts 42 and 44. With this arrangement, the movable contact 36 would be moved outwardly to bridge the semicircular contacts 44 and 42 and the other stationary contacts.

It is also within the contemplation of this invention that the stationary contacts 42 and 44 may be substantially straight instead of semicircular, with their respective stationary contacts located adjacent thereto. With this arrangement, the movable contact 36 would move in a substantially linear or straight direction rather than in a rotating direction.

The surface areas of the contacts may be maximum for the space necessary to contain the contacts by using the contact arrangement shown in FIG. 5. In this arrangement, the contacts 36, 41, 43, 45 and 46 are substantially triangularly shaped. The end projections 86 and 88 of the semicircular contacts 42 and 44 are similarly shaped.

The apparatus and connecting arrangements disclosed herein provide tap selecting means yielding a better flux symmetry pattern in the center voltage rating position than prior art tap selecting means. From flux field plots of a large generator transformer, it has been calculated that the total reduction in losses at the rated voltage would be 23 KW per phase with a tap selecting means constructed according to this invention. Since numerous changes may be made in the above-described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all of the matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative rather than limiting.

I claim as my invention:

1. Electrical inductive apparatus comprising a magnetic core, a winding assembly comprising first and second coils each having a plurality of tap terminals, tap selecting means which electrically connects together two of said tap terminals, said tap selecting means having a plurality of tap selecting positions including center, higher and lower tap selecting positions, said tap selecting means comprising stationary contacts and a movable contact, said movable contact being movable to a plurality of locations corresponding to specific tap selecting positions, said movable contact electrically connecting together two of said stationary contacts at each movable contact location, said stationary contacts being physically arranged and electrically connected to said tap terminals to provide a first tapping pattern when said tap selecting means is set at the center tap selecting position and a second tapping pattern when said tap selecting means is set at the higher tap selecting position, the portion of said first coil which is tapped-out in said first tapping pattern being greater than the portion of said first coil which is tapped-out in said second tapping pattern, said second coil not having any portion thereof tapped-out in said first tapping pattern.

2. The electrical inductive apparatus of claim 1 wherein the stationary contacts are physically arranged and electrically connected to the tap terminals to provide a third tapping pattern when the tap selecting means is set at the lower tap selecting position, the portion of the first coil which is tapped-out in said third tapping pattern being equal to the portion of the first coil which is tapped-out in the first tapping pattern.

3. The electrical inductive apparatus of claim 1 wherein the stationary contacts are physically arranged and electrically connected to the tap terminals so that a portion of the second coil is tapped-out when the tap selecting means is set at the lower tap selecting position.

4. The electrical inductive. apparatus of claim 1 wherein the first coil includes first and second tap terminals, the second coil includes fourth and fifth tap terminals, the tap selecting means includes a second contact connected to said second tap terminal, a fourth contact connected to said fourth tap terminal, a movable contact which is movable with respect to said second and fourth contacts to a position which connects together said second and fourth contacts, a fifth contact connected to said firth tap terminal, said fifth contact positioned so that said movable contact contap selecting means is set at the lower tap selecting position, a first contact connected to said first tap ter minal, said first contact being positioned so that said movable contact connects together said first and fourth contacts when the tapselecting means is set at the higher tap selecting position.

5. The electrical inductive apparatus of claim 4 wherein the tap selecting means includes third and sixth contacts, the first coil includes a third tap terminal which is located between the first and second tap terminals, the second coil includes a sixth tap terminal which is located between the fourth and fifth tap terminals, said third contact being connected to said third tap terminal, and said sixth contact being connected to said sixth tap terminal.

6. Electrical inductive apparatus comprising a magnetic core, a winding assembly comprising a first and second coils, said first coil having first and second tap terminals with a predetermined number of conductorturns positioned between said first and second tap terminals, a third tap terminal connected to one of said conductor-turns of said first coil, said second coil having fourth and fifth tap terminals with a predetermined number of conductor-turns positioned between said fourth and fifth tap terminals, a sixth tap terminal connected to one of said conductor-turns of said second coil, tap selecting means comprising a first stationary stationary contact connected to said second tap terminal, a third stationary contact connected to said third tap terminal, a fourth stationary contact connected to said fourth tap terminal, a fifth stationary contact connected to said fifth tap terminal, said second and fourth stationary contacts each having a substantially semicircular shape with first and second ends, the first ends'of said second and fourth stationary contacts being positioned adjacent to each other, the second ends of said second and fourth stationary contacts being positioned adjacent to each other, said first stationary contact being positioned adjacent to the first end of said fourth stationary contact, said third stationary contact being positioned between said first stationary contact and the second end of said fourth stationary contact, said fifth stationary contact being positioned adjacent to the first end of said second stationary contact, said sixth stationary contact being positioned between said fifth stationary contact and the second end of said second stationary contact, a movable contact sufficiently dimensioned to connect together said first and fourth stationary contacts in a first tap position, to connect together said third and fourth stationary contacts in a second tap position, to connect together said second and fourth stationary contacts in a third tap position, to connect together said second and sixth stationary contacts in a fourth tap position, and to connect together said second and fifth stationary contacts in a fifth tap position. 

1. Electrical inductive apparatus comprising a magnetic core, a winding assembly comprising first and second coils each having a plurality of tap terminals, tap selecting means which electrically connects together two of said tap terminals, said tap selecting means having a plurality of tap selecting positions including center, higher and lower tap selecting positions, said tap selecting means comprising stationary contacts and a movable contact, said movable contact being movable to a plurality of locations corresponding to specific tap selecting positions, said movable contact electrically connecting together two of said stationary contacts at each movable contact location, said stationary contacts being physically arranged and electrically connected to said tap terminals to provide a first tapping pattern when said tap selecting means is set at the center tap selecting position and a second tapping pattern when said tap selecting means is set at the higher tap selecting position, the portion of said first coil which is tapped-out in said first tapping pattern being greater than the portion of said first coil which is tapped-out in said second tapping pattern, said second coil not having any portion thereof tapped-out in said first tapping pattern.
 2. The electrical inductive apparatus of claim 1 wherein the stationary contacts are physically arranged and electrically connected to the tap terminals to provide a third tapping pattern when the tap selecting means is set at the lower tap selecting position, the portion of the first coil which is tapped-out in said third tapping pattern being equal to the portion of the first coil which is tapped-out in the first tapping pattern.
 3. The electrical inductive apparatus of claim 1 wherein the stationary contacts are physically arranged and electrically connected to the tap terminals so that a portion of the second coil is tapped-Out when the tap selecting means is set at the lower tap selecting position.
 4. The electrical inductive apparatus of claim 1 wherein the first coil includes first and second tap terminals, the second coil includes fourth and fifth tap terminals, the tap selecting means includes a second contact connected to said second tap terminal, a fourth contact connected to said fourth tap terminal, a movable contact which is movable with respect to said second and fourth contacts to a position which connects together said second and fourth contacts, a fifth contact connected to said firth tap terminal, said fifth contact positioned so that said movable contact connects together said second and fifth contacts when the tap selecting means is set at the lower tap selecting position, a first contact connected to said first tap terminal, said first contact being positioned so that said movable contact connects together said first and fourth contacts when the tap selecting means is set at the higher tap selecting position.
 5. The electrical inductive apparatus of claim 4 wherein the tap selecting means includes third and sixth contacts, the first coil includes a third tap terminal which is located between the first and second tap terminals, the second coil includes a sixth tap terminal which is located between the fourth and fifth tap terminals, said third contact being connected to said third tap terminal, and said sixth contact being connected to said sixth tap terminal.
 6. Electrical inductive apparatus comprising a magnetic core, a winding assembly comprising a first and second coils, said first coil having first and second tap terminals with a predetermined number of conductor-turns positioned between said first and second tap terminals, a third tap terminal connected to one of said conductor-turns of said first coil, said second coil having fourth and fifth tap terminals with a predetermined number of conductor-turns positioned between said fourth and fifth tap terminals, a sixth tap terminal connected to one of said conductor-turns of said second coil, tap selecting means comprising a first stationary contact connected to said first tap terminal, a second stationary contact connected to said second tap terminal, a third stationary contact connected to said third tap terminal, a fourth stationary contact connected to said fourth tap terminal, a fifth stationary contact connected to said fifth tap terminal, said second and fourth stationary contacts each having a substantially semicircular shape with first and second ends, the first ends of said second and fourth stationary contacts being positioned adjacent to each other, the second ends of said second and fourth stationary contacts being positioned adjacent to each other, said first stationary contact being positioned adjacent to the first end of said fourth stationary contact, said third stationary contact being positioned between said first stationary contact and the second end of said fourth stationary contact, said fifth stationary contact being positioned adjacent to the first end of said second stationary contact, said sixth stationary contact being positioned between said fifth stationary contact and the second end of said second stationary contact, a movable contact sufficiently dimensioned to connect together said first and fourth stationary contacts in a first tap position, to connect together said third and fourth stationary contacts in a second tap position, to connect together said second and fourth stationary contacts in a third tap position, to connect together said second and sixth stationary contacts in a fourth tap position, and to connect together said second and fifth stationary contacts in a fifth tap position. 